I’m seeking help from experts: is there a curve showing the optimal operating range for variable-frequency air conditioners?

I’ve searched on Baidu, but I still don’t find it clear enough; the information available on Baidu seems to contain misleading ads, so I’m seeking answers from professionals. Thank you

I. The main differences between DC variable frequency and AC variable frequency systems lie in their working principles, power consumption, and efficiency: 1. Different working principles: (1) The rotor of an AC variable frequency compressor is based on the structure of an AC induction motor rotor. Its working principle is as follows: the stator generates a rotating magnetic field, and the rotor induces an electric current due to this rotating magnetic field, which in turn creates its own magnetic field; it is the interaction between the stator’s magnetic field and the rotor’s magnetic field that causes the rotor to rotate. (2) The rotor of the DC inverter compressor is made of rare earth permanent magnet materials. Its working principle is as follows: the stator generates a rotating magnetic field that interacts directly with the permanent magnetic field of the rotor, thereby enabling the compressor to operate. 2. Different power consumption: DC inverter systems save 15% on electricity costs compared to AC inverter systems, and they save 45% more energy than conventional machines. However, DC inverter systems are more expensive, and most brands do not produce them, which is why they are not widely used in ordinary households. 3. Different efficiency: DC inverter compressors are 10%-30% more efficient than AC inverter compressors. II. Advantages and Disadvantages 1. AC inverter air conditioners are relatively more power-consuming and produce slightly more noise. The compressors in these air conditioners use AC motors, and their electrical control systems are relatively simple, operating on asynchronous control principles. 2. DC inverter air conditioners: DC inverter systems save 15% on electricity costs compared to AC inverter systems, and they offer advantages in terms of comfort, noise level, lifespan, and control precision. Additional information: Features of inverter air conditioners: 1. Low starting current, with the speed increasing gradually; the starting current is 1/7 that of conventional air conditioners ； 2. There is no issue of temperature fluctuations, as variable-frequency air conditioners gradually reduce their speed as the temperature approaches the set value, eventually reaching that set temperature and operating at a low frequency that balances out heat loss, thus maintaining a stable indoor temperature ； 3. The noise level is lower than that of conventional air conditioners, as inverter air conditioners use dual-rotor compressors, which **reduce rotational imbalance and result in very low vibration in the outdoor unit, approximately half that of conventional air conditioners** ； 4. The cooling and heating speeds are 1–2 times faster than those of conventional air conditioners. Variable-frequency air conditioners use electronic expansion valve throttling technology; the microprocessor can control the degree of opening of the valve based on the information collected by temperature sensors located at various points such as the inlet and outlet of the expansion valve and the compressor’s suction pipe, thereby achieving rapid cooling or heating.

What is referred to as all-direct current inverter technology in the market today means that, in addition to the compressor being a brushless direct current compressor, both the indoor and outdoor fan motors are also brushless direct current motors. Therefore, all-digital inverter technology is more energy-efficient than conventional digital inverter technology (where only the compressor is a brushless DC motor, while the indoor and outdoor fan motors are AC asynchronous motors), but it comes at a relatively higher cost. For air-conditioners with two or fewer compressors, the brushless DC motors used are, compared to AC asynchronous motors, resulting in a total cost of about 100 to 200 yuan higher for both the indoor and outdoor motors. DC frequency conversion involves converting 220V single-phase AC power into 3-phase 400V AC power, while also adjusting the speed through frequency control. Full DC variable frequency conversion involves converting 220V AC electricity directly into DC electricity, and then adjusting the speed by varying the DC voltage. Personally, I think a speed control method that first converts to DC and then adjusts the voltage would be more efficient, as it eliminates the losses associated with inversion. The power consumption difference between full DC variable frequency and conventional DC variable frequency is not significant, estimated to be within 5%, with the main difference lying in aspects such as comfort related to noise.

That is my final answer to this question after more than 2 years. Thank you~